In communications systems in general, and in wireless communication systems in particular, bidirectional connectivity is typically implemented e.g., for application support and/or for control of the communications system. Disruption of connectivity or communication in either direction may lead to failure. Disruption of connectivity must therefore be detected, and appropriate actions to re-establish connectivity be taken.
In the cellular 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system, a connection supervision function may monitor the Downlink (DL) connectivity and User Equipment (UE)-initiated Random Access procedures. Random Access (RA) is a procedure typically used by the UE to request access to the system or resources when the UE discovers a need to acquire Uplink (UL) synchronization or a need to make an uplink transmission and no resources for the uplink transmissions are yet available to the UE. If a problem is detected, a timer is started, at the expiration of which a re-establishment procedure may be triggered. A first timer aims to give the condition an opportunity to be resolved before taking more invasive measures, such as a re-establishment request. A second timer may govern the time during which the UE may attempt a re-establishment if conditions are favourable. If the second timer expires, then the UE autonomously releases the RRC connection and goes to an Idle state.
In addition to UE-initiated RA procedures, LTE supports network-triggered RA procedures. A network-triggered RA procedure is used to force a UE, which does not have a valid uplink timing, to synchronize its UL to the timing at the evolved Node B (eNB). E.g., prior to the eNB making a DL transmission for which the UE will need to transmit Acknowledgment/Non-acknowledgment (ACK/NACK) feedback. Because of the non-zero duration of the RA and UL synchronization procedure, the resynchronization occurs in advance of making the DL transmission.
In 3GPP, it has been agreed that in the event that the RA procedure fails, the entity that initiated the RA procedure is responsible for repeating the procedure. In other words, the UE repeats and supervises a UE-initiated RA procedure and the network (e.g., the eNB) repeats and supervises a network-initiated RA procedure. Thus, if the network detects a problem with a network-initiated RA procedure, the network has the responsibility of appropriately instructing the UE to release or initiate re-establishment of the connection.
In communications systems, such as LTE, where data and control messages are encrypted, the UE and the network can only understand each other if they share a common view of the security parameters, including, for example, encryption and integrity protection algorithms, keys, counters, etc. Situations may exist where the security parameters available in the UE do not match the security parameters available in the network. For example, security parameters can be inaccurate or missing (e.g., due to a Handover (HO) where the UE goes to a target cell different from the cell that was prepared by the source eNB).
If the network, due to missing or inaccurate security parameters, cannot instruct the UE to release the connection or initiate connection re-establishment and the same instructions, for security reasons, cannot be sent unciphered and/or without integrity protection, the UE will be stuck in a state of broken connectivity.